Subacromial Impingement | Neer Stages | Cuff Compression
NEER CLASSIFICATION
Critical Must-Knows
- External impingement = compression of supraspinatus between humeral head and acromion
- Neer stages progress from edema (I) to fibrosis (II) to cuff tear (III)
- Acromial morphology (Type 3 hooked) increases impingement risk
- Non-operative treatment first-line for Stage I-II
- Acromioplasty removes impinging bone but evidence is debated
Clinical Pearls
- "Bigliani Type 3 (hooked) acromion = higher impingement risk
- "Hawkins and Neer tests assess subacromial impingement
- "Differentiate from internal impingement (posterior/superior, throwers)
- "Subacromial injection test helps confirm diagnosis
Clinical Imaging
Imaging Gallery
Critical External Impingement Exam Points
Neer Stages
Stage I: Edema and hemorrhage (under 25 years, reversible). Stage II: Fibrosis and tendinitis (25-40 years, chronic). Stage III: Rotator cuff tear (over 40 years, often irreversible). Progression is not inevitable.
Acromial Morphology
Bigliani classification: Type 1 (flat), Type 2 (curved), Type 3 (hooked). Type 3 has highest impingement risk. Outlet view X-ray or sagittal MRI shows acromion shape.
External vs Internal
External = subacromial, anterior cuff compression, older patients. Internal = posterior-superior, articular side cuff, throwers/overhead athletes in abduction-ER. Know the distinction.
Acromioplasty Evidence
Recent RCTs (CSAW, FIMPACT) show acromioplasty may not be superior to sham surgery for isolated impingement. This is controversial but important to know for examination.
External vs Internal Impingement
| Feature | External (Subacromial) | Internal (Posterior-Superior) |
|---|---|---|
| Location | Subacromial space, anterior | Posterior-superior, articular |
| Cuff surface | Bursal side | Articular side |
| Age group | Over 35, degenerative | Under 35, throwers/athletes |
| Mechanism | Cuff compressed under acromion | Cuff pinched on glenoid rim in abd-ER |
| Treatment | Physio, injection, acromioplasty | Physio, posterior capsule stretch |
EFTNeer Stages
| E | Edema (Stage I) Under 25 years, reversible with rest |
| F | Fibrosis (Stage II) 25-40 years, chronic tendinitis |
| T | Tear (Stage III) Over 40 years, cuff rupture |
| E | Edema (Stage I) Under 25 years, reversible with rest |
| F | Fibrosis (Stage II) 25-40 years, chronic tendinitis |
| T | Tear (Stage III) Over 40 years, cuff rupture |
Hook:EFT = Edema, Fibrosis, Tear - Neer stages progress with age and chronicity!
1-2-3Bigliani Acromion Types
| 1 | Flat Lowest impingement risk |
| 2 | Curved Moderate risk |
| 3 | Hooked Highest impingement risk |
| 1 | Flat Lowest impingement risk |
| 2 | Curved Moderate risk |
| 3 | Hooked Highest impingement risk |
Hook:Bigliani 1-2-3: Flat, Curved, Hooked - Type 3 is trouble!
NHJClinical Tests
| N | Neer test Forward flexion with scapula stabilized |
| H | Hawkins test 90° flexion with internal rotation |
| J | Jobe test Empty can for supraspinatus (not impingement-specific) |
| N | Neer test Forward flexion with scapula stabilized |
| H | Hawkins test 90° flexion with internal rotation |
| J | Jobe test Empty can for supraspinatus (not impingement-specific) |
Hook:NHJ = Neer, Hawkins, Jobe - key shoulder tests!
Overview and Epidemiology
Why External Impingement Matters
External (subacromial) impingement is one of the most common causes of shoulder pain. Understanding the spectrum from edema to cuff tear, the role of acromial morphology, and the evidence around treatment options is essential for examination and clinical practice.
Demographics
- Over 35 years typical onset
- Occupational overhead workers
- Athletes (overhead sports)
- Degenerative component with aging
- Both genders equally affected
Risk Factors
- Type 3 hooked acromion
- Os acromiale
- Acromioclavicular osteophytes
- Overhead repetitive activity
- Postural factors (kyphosis)
Pathophysiology and Mechanisms
Subacromial Space Anatomy
The subacromial space lies between the acromion/coracoacromial ligament above and the humeral head below. The supraspinatus tendon passes through this space. In external impingement, this space is narrowed, causing mechanical compression of the rotator cuff, particularly during forward flexion and internal rotation.
Causes of Subacromial Narrowing
| Extrinsic (From Above) | Intrinsic (Within Tendon) |
|---|---|
| Type 3 hooked acromion | Tendon degeneration |
| AC joint osteophytes | Tendon calcification |
| Os acromiale | Cuff tear (partial/full) |
| Coracoacromial ligament thickening | Tendon thickening from tendinitis |
Coracoacromial Arch
- Acromion - anterior-superior coverage
- Coracoacromial ligament - connects acromion to coracoid
- Coracoid process - anterior
- This arch forms the "roof" that can impinge
Impingement Zone
- Critical zone of supraspinatus (1cm from insertion)
- Watershed area - relatively avascular
- Bursal surface affected in external impingement
- Articular surface affected in internal impingement
Classification Systems
Neer Classification of Impingement
| Stage | Pathology | Age Group | Treatment |
|---|---|---|---|
| Stage I | Edema and hemorrhage | Under 25 years | Rest, physio, NSAIDs |
| Stage II | Fibrosis and tendinitis | 25-40 years | Physio, injection, +/- surgery |
| Stage III | Rotator cuff tear | Over 40 years | Decompression +/- cuff repair |
Neer's classification emphasizes the progressive nature of impingement syndrome, though progression is not inevitable.
Clinical Assessment
History
- Anterior-lateral shoulder pain
- Night pain (classic)
- Overhead activity exacerbates
- Painful arc during abduction
- Weakness if cuff involved
Examination
- Neer test - forward flex with scapula fixed
- Hawkins test - 90° flex, internal rotate
- Painful arc - 60-120° abduction
- Jobe test - supraspinatus strength
- Injection test - relief with subacromial LA
Clinical Tests Explained
Neer test: Examiner stabilizes scapula, passively forward flexes arm. Positive if reproduces pain (compresses cuff against anterior acromion).
Hawkins test: Flex shoulder/elbow to 90°, then internally rotate forearm. Positive if reproduces pain (compresses cuff against coracoacromial ligament).
Subacromial injection test: Inject LA into subacromial space. If pain relief and improved strength, supports impingement diagnosis.
Investigations
Investigation Protocol
AP, axillary, outlet (Y) views. Assess acromial morphology on outlet view. Look for AC joint arthritis, calcific tendinitis, superior migration of humeral head.
Dynamic assessment. Can visualize cuff tears, bursitis. Operator-dependent. Less detail than MRI.
Best for cuff assessment. Shows tendon pathology, tears, muscle quality. Sagittal views show acromial morphology. Reserve for suspected cuff tear or failed conservative treatment.
Outlet View
The supraspinatus outlet view (scapular Y-view) best demonstrates acromial morphology. Look for Type 3 hooked acromion, subacromial spurs, and os acromiale. This view is essential for impingement assessment.
Management Algorithm
First-Line Treatment
Stage I and most Stage II are treated non-operatively:
1. Activity modification:
- Avoid aggravating overhead activities
- Relative rest (not complete immobilization)
2. Physiotherapy:
- Rotator cuff strengthening
- Scapular stabilization exercises
- Posterior capsule stretching
- Posture correction
3. NSAIDs:
- Short-term for pain relief
- Helps reduce inflammation
4. Subacromial corticosteroid injection:
- Provides temporary relief
- Aids physiotherapy participation
- Max 3 injections per year
- Diminishing returns with repeated use
Success rate for non-operative treatment is 70-90% if compliant with physiotherapy.
Pre-operative Planning
Imaging Review
- Confirm acromial morphology
- Assess for cuff tear (MRI if needed)
- AC joint status
- Exclude other pathology
- Os acromiale evaluation
Surgical Planning
- Beach chair or lateral position
- Standard posterior/lateral portals
- Plan bursectomy extent
- Acromioplasty targets
- Consent includes limited evidence
Surgical Technique
Subacromial Decompression
Surgical Steps
Beach chair or lateral decubitus. Arm in holder with traction if lateral. Standard portals (posterior viewing, lateral working).
First evaluate the glenohumeral joint. Assess rotator cuff (articular side), biceps, labrum. Enter subacromial space.
Remove inflamed bursal tissue with shaver. Visualize undersurface of acromion, coracoacromial ligament, and rotator cuff.
Use burr to flatten anterior-inferior acromion. Remove impinging spurs. Aim for Type 1 (flat) morphology. Do not over-resect.
Release coracoacromial ligament if thickened/impinging. Some preserve to maintain coracoacromial arch integrity.
Avoid Over-Resection
Excessive acromial resection can cause fracture or deltoid detachment. Remove only enough to create flat undersurface. The coracoacromial ligament provides superior restraint - consider preserving if possible.
Complications
| Complication | Incidence | Risk Factors | Prevention/Management |
|---|---|---|---|
| Persistent pain | 10-30% | Patient selection, technique | Proper indication, thorough decompression |
| Acromial fracture | Rare | Excessive resection | Conservative bone removal |
| Deltoid detachment | Rare | Aggressive resection | Protect deltoid origin |
| Stiffness | 5% | Inadequate rehab | Early ROM protocol |
CSAW Trial Implications
The CSAW trial (UK, 2018) showed no significant difference between arthroscopic subacromial decompression and sham surgery at 6 months. This has led to questioning of routine ASD for isolated impingement. Patients should be informed of this evidence during consent.
Postoperative Care and Rehabilitation
Rehabilitation After ASD
Recovery Timeline
Sling comfort only. Begin pendulum exercises. Ice for swelling. May remove sling for exercises.
Active-assisted ROM progressing to active. Begin rotator cuff isometrics. Scapular stabilization.
Progressive strengthening. Rotator cuff and deltoid focus. Return to light activities.
Full return to activity. Sport-specific training. Most recovery by 3 months.
Recovery from isolated ASD is relatively quick compared to cuff repair.
Outcomes and Prognosis
Treatment Outcomes
| Treatment | Success Rate | Notes |
|---|---|---|
| Physiotherapy alone | 70-90% | First-line for Stage I-II |
| Corticosteroid injection | 50-70% at 6 weeks | Temporary benefit, aids PT |
| ASD (isolated impingement) | 65-85% | Debated benefit vs sham |
| ASD + cuff repair | 75-90% | Benefit from cuff repair |
Evidence Controversy
CSAW (UK, 6-month primary endpoint with concordant 1-year data) and FIMPACT (Finland, 24-month follow-up) both found no clinically important benefit of ASD over placebo arthroscopy for impingement with an intact cuff. The 2019 Cochrane review (1062 patients) and the BMJ Rapid Recommendation graded this as high-certainty evidence and recommended against routine ASD. The remaining debate is patient selection and whether a structural cuff tear changes the calculus - be prepared to discuss in examinations.
Controversies & Areas of Uncertainty
Does the operation work?
- High-certainty RCT/Cochrane evidence: ASD gives no clinically important benefit over placebo surgery for impingement with an intact cuff
- Improvement after surgery is largely placebo plus rehabilitation
- "Impingement" is shifting to the broader, mechanism-neutral term subacromial pain syndrome
Cause vs effect of acromial shape
- Type 3 acromion associates with cuff tears, but causation is unproven
- Hooks may be traction enthesophytes (effect), not a primary cause
- Bigliani classification has poor inter-observer reliability
Extrinsic vs intrinsic
- Neer's extrinsic compression model is now balanced against intrinsic tendinopathy (age, vascularity, load)
- Most contemporary models are multifactorial
Where surgery may still help
- Genuine structural lesions (e.g. large anterior spur, symptomatic os acromiale) - not pain alone
- Decompression as an adjunct during rotator-cuff repair remains accepted practice
Evidence Base and Key Trials
CSAW Trial - ASD vs Placebo Arthroscopy vs No Treatment
- Multicentre UK 3-arm placebo-controlled RCT, 313 patients, 32 hospitals
- Oxford Shoulder Score equivalent for decompression vs arthroscopy-only placebo (mean diff -1.3, NS)
- Both surgical arms beat no treatment by a margin too small to be clinically important
- Difference over no treatment likely placebo effect and/or postoperative physiotherapy
FIMPACT Trial - ASD vs Diagnostic Arthroscopy vs Exercise
- Multicentre Finnish 3-arm RCT, 210 patients, 24-month follow-up
- No clinically relevant difference in pain (VAS) between ASD and diagnostic (placebo) arthroscopy
- ASD vs diagnostic arthroscopy difference under the 15-point MCID at rest and on activity
- Apparent ASD advantage over exercise did not exceed the MCID and was biased by selective dropout
Cochrane Review - Subacromial Decompression for Rotator Cuff Disease
- 8 RCTs, 1062 participants with impingement (full-thickness tears excluded)
- High-certainty evidence: ASD gives no improvement in pain, function or quality of life vs placebo at 1 year
- Mean pain difference 0.26 points (0-10 scale) favouring placebo arm - not clinically important
- Serious adverse-event risk after shoulder arthroscopy likely under 1%
Lähdeoja Meta-analysis - Basis for BMJ Rapid Recommendation
- Systematic review with meta-analysis underpinning the BMJ Rapid Recommendations panel
- High-certainty: no benefit of ASD over placebo surgery for pain at 1 year (MD -0.26, MID 1.5)
- Moderate-to-high certainty: no benefit for function or health-related quality of life
- Approximately 6 serious harms per 1000 patients undergoing ASD
Neer Original Description of Impingement
- Coined the impingement syndrome concept and three progressive stages
- Described anterior acromioplasty as the operative remedy
- Located impingement at the anterior third of the acromion and CA ligament
- Volume 54-A, pages 41-50 - the foundational paper of the field
Bigliani Acromial Morphology Classification
- Defined three acromial undersurface shapes: flat, curved, hooked
- Type 3 (hooked) acromion most strongly associated with full-thickness cuff tears in cadavers
- Best profiled on the supraspinatus outlet (scapular-Y) view
- Widely adopted descriptive classification
Kuhn - Exercise for Rotator Cuff Impingement (Evidence-Based Protocol)
- Systematic review of 11 level 1-2 RCTs of exercise for impingement
- Exercise produces statistically and clinically significant pain reduction and functional gain
- Manual therapy augments exercise; supervised and home programmes perform similarly
- Synthesised into a standard evidence-based rehabilitation protocol
Exam Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Impingement Assessment (~2-3 min)
"A 45-year-old office worker presents with 6 months of right shoulder pain, worse at night and with overhead activities. How do you assess for subacromial impingement?"
Scenario 2: Treatment Approach (~2-3 min)
"A 50-year-old man has Neer Stage II impingement confirmed clinically. MRI shows thickened supraspinatus tendon without tear. He has failed 3 months of physiotherapy. What is your management?"
Scenario 3: Acromial Morphology (~2 min)
"Describe the Bigliani classification of acromial morphology and its clinical significance."
MCQ Practice Points
Neer Classification Question
Q: What pathology is seen in Neer Stage II impingement? A: Fibrosis and tendinitis - Stage I is edema/hemorrhage (under 25 years), Stage II is fibrosis/tendinitis (25-40 years), Stage III is rotator cuff tear (over 40 years).
Bigliani Question
Q: Which Bigliani acromial type has the highest impingement risk? A: Type 3 (Hooked) - Type 1 is flat (lowest risk), Type 2 is curved (moderate risk), Type 3 is hooked (highest risk and associated with cuff tears).
Clinical Test Question
Q: How is the Neer impingement test performed? A: Forward flex the arm with scapula stabilized - The examiner stabilizes the scapula and passively forward flexes the arm overhead. This compresses the cuff against the anterior acromion. Positive if reproduces the patient's pain.
Hawkins Test Question
Q: How is the Hawkins impingement test performed? A: Flex shoulder and elbow to 90°, then internally rotate - The arm is positioned in 90° of forward flexion and elbow flexion, then the forearm is internally rotated. This compresses the cuff against the coracoacromial ligament.
X-ray View Question
Q: Which X-ray view best demonstrates acromial morphology? A: Outlet view (Supraspinatus outlet/Scapular Y-view) - This view shows the acromial profile and allows classification into Bigliani Types 1, 2, or 3. Also visible on sagittal MRI.
CSAW Trial Question
Q: What did the CSAW trial show regarding arthroscopic subacromial decompression? A: ASD was not significantly better than sham surgery at 6 months for isolated subacromial impingement without rotator cuff tear. This has led to debate about routine ASD for impingement.
Guidelines, Registries & Global Practice
Global epidemiology. Shoulder pain is among the three most common musculoskeletal presentations worldwide; subacromial pain syndrome (the contemporary umbrella term for external impingement with an intact cuff) accounts for the majority of these consultations. Prevalence rises with age and overhead occupational or sporting load and is broadly similar across high- and limited-resource settings.
Side-by-side guideline positions on subacromial decompression
| Body / region | Position on routine ASD for impingement (intact cuff) |
|---|---|
| BMJ Rapid Recommendations (international panel, MAGIC/GRADE) | Strong recommendation AGAINST ASD; offer structured exercise first |
| NICE / BOA (UK) | Conservative care first-line; surgery not for isolated impingement without clear structural cause |
| AAOS (US) | Emphasises non-operative management; shared decision-making given equivocal surgical evidence |
| AO Foundation / EFORT (Europe) | Reserve decompression for documented mechanical/structural lesions, not pain alone |
Registry & Evidence Signal
- Cochrane 2019 (1062 patients): high-certainty no benefit of ASD vs placebo
- Declining ASD rates reported in several health systems post-CSAW/FIMPACT
- Serious harm after shoulder arthroscopy likely under 1%
- Convergent guidance across UK, US and Europe
High vs Limited-Resource Practice
- Well-resourced settings: MRI access, arthroscopy available but increasingly restrained
- Limited-resource settings: clinical diagnosis plus injection test, supervised/home exercise
- Exercise (supervised or home) performs similarly - key for low-resource equity
- Acromial morphology assessed on plain outlet view where MRI is scarce
Documentation & Consent (medicolegal)
Key documentation requirements:
- Document failed conservative treatment (type, duration, compliance)
- Record clinical examination with specific tests
- Note imaging findings (acromial morphology, cuff status)
- Consent must discuss: CSAW/FIMPACT evidence (ASD may not be better than sham), alternatives, expected outcomes, risks
- If proceeding to surgery, document patient's informed decision
EXTERNAL IMPINGEMENT
Clinical summary
Definition
- •Compression of supraspinatus under acromion
- •External = subacromial (vs internal = posterior-superior)
- •Bursal side cuff affected
- •Degenerative/mechanical etiology
Neer Classification
- •Stage I: Edema/hemorrhage (under 25 years)
- •Stage II: Fibrosis/tendinitis (25-40 years)
- •Stage III: Rotator cuff tear (over 40 years)
Bigliani Acromion Types
- •Type 1: Flat (lowest risk)
- •Type 2: Curved (moderate risk)
- •Type 3: Hooked (highest risk)
- •Seen on outlet view X-ray
Clinical Tests
- •Neer: Forward flex with scapula fixed
- •Hawkins: 90° flex, internal rotate
- •Painful arc: 60-120° abduction
- •Injection test: LA into subacromial space
Treatment
- •Non-operative first (70-90% success)
- •Physio, NSAIDs, injection
- •ASD debated (CSAW/FIMPACT trials)
- •Surgery if cuff tear present
CSAW Trial
- •ASD vs sham surgery comparison
- •No significant difference at 6 months
- •Questions routine ASD for isolated impingement
- •Must discuss in consent